1. Field of the Invention
The present invention relates to an electrolyte membrane for a fuel cell.
2. Description of the Related Art
Fuel cells directly convert chemical energy into electric energy by supplying a fuel and an oxidizing agent to two electrodes that are electrically connected and electrochemically inducing oxidation of the fuel. By contrast with thermal power generation, fuel cells are not affected by the limitations of Carnot cycle and, therefore, demonstrate a high energy conversion efficiency. A fuel cell is usually configured by stacking a plurality of unit cells containing as a basic structure a membrane-electrode assembly (MEA) in which an electrolyte membrane is sandwiched between a pair of electrodes. Among such fuel cells, fuel cells of a solid polymer electrolyte type (PEFC) that use a solid polymer electrolyte membrane as the electrolyte membrane attracted attention as power sources, in particular, for portable devices and movable bodies, because such fuel cells have a number of advantages including the easiness of miniaturization and operability at a low temperature.
In a fuel cell of a solid polymer electrolyte type, when hydrogen is used as a fuel, a reaction represented by Equation (1) below proceeds at an anode (fuel electrode).H2→2H++2e−  (1)
Electrons generated according to Equation (1) perform a work in an external load via an external circuit and then reach a cathode (oxidizing agent electrode). Protons generated according to Equation (1) move by electro osmosis from the anode to the cathode inside the solid polymer electrolyte membrane in a state of hydration with water.
Further, when oxygen is used as an oxidizing agent, a reaction represented by Equation (2) below proceeds at the cathode.2H++(½)O2+2e−→H2O  (2)
Water generated at the cathode mainly passes through a gas diffusion layer and is discharged to the outside. Thus, fuel cells are clean power generating devices producing no wastes other than water.
A polymer electrolyte membrane that can operate in a temperature range of fuel cells of a solid polymer electrolyte membrane type that are usually used is composed of a proton conductive material of an organic polymer type that has a polymer in a basic skeleton or main chain. Dimensional changes such as expansion and contraction of the membrane during water absorption and desorption and the occurrence of heat-induced creep or thermal shrinkage is a problem associated with such polymer conductive materials. In the operation environment of fuel cells, the water and heat balance is known to change frequently due to a load or external environment, and dimensional changes of the membrane caused by such changes can shorten the electrolyte service life. This is one more very serious problem associated with the presently available polymer conductive materials of an organic polymer type.
On the other hand, an electrolyte membrane combining an inorganic proton conductor and a non-proton conductive polymer has been suggested, this electrolyte membrane being different from the above-described electrolyte membrane using a proton conductive material of an organic polymer type. Japanese Patent Application Publication No. 2002-289051 (JP-A-2002-289051) disclosed a proton conductive membrane including a metal oxide hydrate represented by a tungsten oxide hydrate or a tin oxide hydrate and a non-proton conductive polymer in order to maintain stable proton conductivity and mechanical strength even at a temperature equal to or greater than 100° C., which is heat resistance limit of fluorine-containing electrolyte membranes.
The proton conductive membrane disclosed in JP-A-2002-289051 has stable proton conductivity at a high temperature and under low-humidity conditions when used as an electrolyte membrane for a fuel cell. However, as compared with Nafion, which is an organic polymer of related art, the proton conductivity value is by itself insufficient under operation conditions (temperature, humidity) at which the power generation efficiency is the best for a fuel cell. Further, the invention of JP-A-2002-289051 is not concerned with dimensional stability of electrolyte membranes.